Method and structure for radiation hardening a semiconductor device

ABSTRACT

Semiconductor devices can be fabricated using conventional designs and process but including specialized structures to reduce or eliminate detrimental effects caused by various forms of radiation. Such semiconductor devices can include the one or more parasitic isolation devices and/or buried guard ring structures disclosed in the present application. The introduction of design and/or process steps to accommodate these novel structures is compatible with conventional CMOS fabrication processes, and can therefore be accomplished at relatively low cost and with relative simplicity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/346,319, filed Jan. 9, 2012, entitled METHOD FOR RADIATION HARDENINGA SEMICONDUCTOR DEVICE, now U.S. Pat. No. 8,497,195, issued Jul. 30,2013, which is a divisional of U.S. patent application Ser. No.11/949,708, filed Dec. 3, 2007, entitled METHODS FOR OPERATING ANDFABRICATING A SEMICONDUCTOR DEVICE HAVING A BURIED GUARD RING STRUCTURE,now U.S. Pat. No. 8,093,145, issued Jan. 10, 2012, which is acontinuation of U.S. patent application Ser. No. 10/951,283, filed Sep.27, 2004, entitled BURIED GUARD RING AND RADIATION HARDENED ISOLATIONSTRUCTURES AND FABRICATION METHODS, now U.S. Pat. No. 7,304,354, issuedDec. 4, 2007, which claims benefit of U.S. Provisional Application No.60/545,271, filed Feb. 17, 2004, entitled BURIED GUARD RING ANDRADIATION HARDENED ISOLATION STRUCTURES AND FABRICATION METHODS, nowexpired, the specifications of which are incorporated herein in theirentirety.

TECHNICAL FIELD

The present invention relates to integrated circuits, and particularlyto integrated circuits with partial and/or complete immunity to failuremodes associated with radiation exposure.

BACKGROUND

The high radiation environments, including the upper atmosphere,near-earth orbit, outer space, and certain terrestrial environments(e.g., proximity to devices producing significant amounts of radiation)provide the most hostile environments for reliable operation ofmicroelectronic solid-state devices. Exposure to radiation causeselectrical degradation of both transistors and circuit-isolationelements, which can lead to sporadic device behavior and/or completedestructive failure of integrated circuits (ICs). Because of thecomplexities of designing and fabricating integrated circuits tolerantof radiation environments, during the 1980s a number of large commercialsemiconductor companies began to specialize in the production ofradiation-hardened ICs, primarily for military and aerospace systems.

At the same time, the high manufacturing costs of non-radiation-hardenedcommercial ICs has generally been offset by progress in high volumeproduction, growing from less than $40B to more than $200B in 2004. Toremain competitive, commercial IC manufacturers have deployed newstate-of-the art silicon IC manufacturing facilities every 3-5 years.

The more limited low-volume demand for radiation-hardened ICs cannotjustify the expense of dedicated leading-edge manufacturing facilities,despite the very attractive margins in the military and aerospaceelectronics market. These financial constraints have severely limitedthe ability of radiation-hardened IC suppliers to utilize leading-edgeIC manufacturing technology. Consequently, the number of companiesproducing radiation-hardened IC components has been dramaticallyreduced, and their capabilities have fallen far behind those of thecommercial sector.

During the 1990s, the combination of rising costs for new ICmanufacturing facilities, military budget reductions, and a dwindlingnumber of suppliers widened the technological disparity betweencommercial and radiation-hardened microelectronics. Commercial andmilitary satellite manufacturers attempting to bridge this gap wereforced to employ a new concept called “COTS” (commercial off-the-shelf)to procure the high-performance ICs required for building theirelectronic platforms.

The COTS approach uses extensive laboratory testing of commodity(unhardened) commercial ICs to screen and “qualify” them forapplications where they are likely to be exposed to damaging radiation.COTS was considered the only practical solution to obtainspace-qualified high-performance ICs. Despite greatly diluted radiationstandards, qualified product could not reliably be found using COTS.Therefore, those few nominally acceptable ICs typically offered nosignificant cost savings. Satellites manufactured using COTS ICs havesuffered significant reductions in capability and mission lifetime dueto destructive radiation exposure. The advancing miniaturization of CMOStechnology increases sensitivity to certain forms of radiation, furtherwidening the gap between COTS capabilities and space electronics marketrequirements. After more than ten years of system failures, the COTSapproach has failed to provide a viable solution for the supply ofradiation-hardened ICs.

Accordingly, it is desirable to bring high-performance andcost-effective radiation-hardened integrated circuits (RHICs) tomilitary, aerospace, and certain terrestrial electronics markets usingthe readily accessible leading-edge infrastructure of high-volumecommercial microelectronics manufacturers. More specifically, it isfurther desirable to systematically address at the silicon process leveleach of the degradation mechanisms caused by radiation and to therebydevelop new radiation hardened solutions that can be integrated intocommercial microelectronic fabrication processes without impactingsignificantly the commercial baseline electrical spice parameters. Thismethodology offers the promise of circuit intellectual property (IP)re-use which would create new and distinct radiation hard circuitproducts from existing commercial circuit designs while avoiding costlycircuit redesigns.

SUMMARY

It has been discovered that semiconductor devices can be fabricatedusing conventional designs and process but including specializedstructures to reduce or eliminate detrimental effects caused by variousforms of radiation. Such semiconductor devices can include the one ormore parasitic isolation devices and/or buried guard ring structuresdisclosed in the present application. The introduction of design and/orprocess steps to accommodate these novel structures is compatible withconventional CMOS fabrication processes, and can therefore beaccomplished at relatively low cost and with relative simplicity.

Accordingly, one aspect of the present invention provides a method whichincludes: exposing an operating semiconductor device to ionizingradiation, and thereby generate charge within a substrate region;collecting the charge onto a buried layer formed within the substrateregion and below at least a portion of a first well and a second well,each of said first and second wells having a respective transistortherein; shunting the collected charge from the buried layer to asurface terminal through a conductive path comprising a conductiveregion disposed between the buried layer and the surface terminal, saidconductive path having a higher conductance than otherwise would beprovided by the first well and the substrate region in the absence ofsaid conductive region; and dispersing the collected charge from thesurface terminal into a voltage source operably coupled to the surfaceterminal.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. As willalso be apparent to one skilled in the art, the operations disclosedherein may be implemented in a number of ways, and such changes andmodifications may be made without departing from this invention and itsbroader aspects. Other aspects, inventive features, and advantages ofthe present invention, as defined solely by the claims, will becomeapparent in the non-limiting detailed description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description andthe accompanying drawings, in which like reference numbers indicate likefeatures.

FIGS. 1A-1H illustrate simplified block diagrams of a transistorstructure utilizing one of the techniques of the present invention;

FIG. 2 shows a cross-section diagram of a CMOS inverter formed on ap−substrate;

FIG. 3 illustrates a simplified block diagram of an inverter structureutilizing several of the techniques and devices of the presentinvention;

FIG. 4 illustrates a simplified block diagram of an inverter structureutilizing additional techniques and devices of the present invention;

FIG. 5 illustrates a simplified block diagram of an inverter structureutilizing still other techniques and devices of the present invention;

FIGS. 6A-6B illustrate additional embodiments of the structureillustrated in FIG. 3;

FIGS. 7A-7C illustrate other embodiments of the present invention;

FIGS. 8A-8E illustrate still other embodiments of the present invention;

FIGS. 9A-9C illustrate 2D and 1D simulations of the doping profiles ofstructures utilizing several of the techniques of the present invention;

FIG. 10 provides a typical current vs. voltage (I-V) trace illustratinglatch-up;

FIG. 11 illustrates I-V traces showing latch-up improvement associatedwith several of the techniques of the present invention; and

FIGS. 12-13 illustrate 1D simulations of doping profiles showingrespectively an exemplary HDBL below a p+ diffusion in an n-well, andbelow a p+ diffusion in a p− well, for an exemplary process.

DETAILED DESCRIPTION

The following sets forth a detailed description of at least the bestcontemplated mode for carrying out the one or more devices and/orprocesses described herein. The description is intended to beillustrative and should not be taken to be limiting.

The devices and techniques described in the present application candeliver a complete radiation solution, or in some cases a specializedradiation solution, by directly addressing one or more of the primarydegradation radiation effects of ICs: Total Ionizing Dose (TID), SingleEvent Latch-up (SEL), and Single-Event Upset (SEU). Moreover, thesolutions to these degradation mechanisms are accomplished at thesilicon process level with modifications to existing foundry baselineprocess. These unique approaches can be integrated into any CMOS processflow without affecting the performance of the underlying baselinetechnology. These approaches can further take advantage of a variety ofdifferent IC fabrication technologies, such as silicon-on-insulator(SOI) and the like. Additionally, although the examples of the presentapplication will focus on MOS (and particularly CMOS) implementations,it will be understood by those having ordinary skill in the art that thedevices and techniques disclosed herein can be extended to othersemiconductor architectures such as BiCMOS, etc.

This disclosure describes various structures and techniques that eachgenerally provide some manner of protection from one or more of thedegradation effects associated with radiation, and particularly ionizingradiation. When combined, these structures and techniques provide novelradiation hardened process isolation (RHPI) schemes. When integratedinto a CMOS process silicon structure, the RHPI structures cansignificantly reduce the sensitivity of the silicon electrical devicesto ionizing radiation, single event latch up, and single event softerror. The RHPI structures can generally be applied to any MOS or BiCMOSsilicon process and will directly improve radiation hardening propertiesof the integrated circuit both during and after exposure to a radiationenvironment. Moreover, this is accomplished with little or no penalty tocircuit size and or electrical performance. As such, the RHIP schemescan be used as an “add on” feature, which can be applied to existing MOSor BiCMOS process technology, improving radiation isolation propertieswithout the need for extensive process or circuit changes. The RHPIstructures enable radiation hardening of commercial digital process byleveraging existing solid state processing technology and re-use ofleading edge circuit IP for the creation of radiation hardened circuitproduct.

Ionizing radiation occurs naturally in the form of high-energy photonsor charged particles that possess enough energy to break atomic bondsand create electron hole pairs in an absorbing material. These particlescan include protons, electrons, atomic ions, and photons with energiesgreater than a bandgap of the absorbing material. When typicalintegrated circuits, such as CMOS integrated circuits, are exposed tothe charged particles over a period of months or even years, theionizing radiation can contribute to a total ionizing dose (TID). Forexample, as an ionizing particle passes through a MOS device, itgenerates one or more electron-hole pairs which can be trapped in thegate oxides and the field oxides. Electrons in the semiconductor'svalence band are raised to the conduction band. A fraction of theelectron-hole pairs will undergo initial recombination and cause nodamage, but such recombination decreases as the electric fieldincreases, and the electrons and holes that survive it are free todiffuse and drift within the oxide where they will be swept from theinsulator, recombine, or be trapped.

Mobile electrons typically move through both gate and field oxidesquickly, but the holes have a relatively low effective mobility and areeasily trapped in gate oxides and field oxides. Because the trappedholes create a positive oxide charge, the resulting space-charge fieldscause negative shifts in the threshold voltages of correspondingtransistors. As a threshold voltage shifts negatively, a transistorbiased in the off-state lets more and more current pass. If enough holesare trapped, an n-channel transistor will remain fully conducting evenwith zero applied gate bias, causing, for example, an enhancement-modedevice to become a depletion-mode device. In many cases, the edge orfield region of the device becomes depletion-mode. Should that happen,leakage currents passing around a device (or from device to device) cancause parameter degradation and, ultimately, device and circuit failure.

Ionizing radiation also boosts the interface trap density in MOSstructures. Interface traps are localized electronic states close to theinterface between silicon and silicon dioxide and can exchange chargewith the silicon conduction and valence bands. They shift thresholdvoltage and can also degrade mobility by acting as scattering centers.More specifically, the increase in interface states shifts the thresholdvoltages in the negative direction (more depletion V_(t)) for n-channeldevices and in the positive direction (more enhancement V_(t)) forp-channel devices. In addition to the positive oxide shift and theinterface states shift described above with respect to n-channel andp-channel devices, threshold voltage shifts caused by ionizing radiationfurther affect parasitic MOS elements, giving rise, for example, toleakage currents that may cause parametric failure and/or functionalfailure.

Even more specialized devices can be susceptible to TID. For example,SOI transistors can also be affected by TID through charge buildup inthe insulator such as a buried oxide. For partially depletedtransistors, charge buildup can invert the back surface causing asource-to-drain (back-channel) leakage current. Moreover, this leakagecurrent is relatively unaffected by gate bias. For fully-depletedtransistors, there is significant sensitivity to radiation-induced oxideand interface-trap charge buildup. Positive oxide charge buildup tendsto deplete the back-channel interface causing a decrease in thethreshold voltage of the front-channel gate oxide and increasedback-channel leakage current. Thus, a wide variety of threshold voltageinstabilities can result.

Thus, the effects of ionizing radiation lead to the degradation ofperformance and ultimate failure of the CMOS devices. The additionalradiation-induced interface states degrade the circuit performance byreducing the channel mobility, which as a result decreases channelconductance and transistor gain. Over time, the threshold voltages ofthe n-channel and p-channel devices may shift to such a degree where then-channel transistors cannot be turned off and the drive capability ofthe p-channel transistors is not sufficient for the circuit to continueoperating at the system clock rate. Such a shift in threshold voltagesof either the n-channel or p-channel transistors can cause a circuit tofail.

FIGS. 1A-1H illustrate one approach to addressing the parasitic effectsassociated with TID in a conventional MOS device. Transistor 100 (shownin FIGS. 1B and 1D-H) differs from conventional MOS devices (shown inFIGS. 1A and 1C) in that channel region 130 of transistor 100 includestwo channel region extensions 135. In traditional MOS devices liketransistor 50, source (60), drain (70), and channel regions (80) aretypically fabricated to have approximately the same width as shown.However, as shown in FIGS. 1B and 1D-H, source region 110 and drainregion 120 have approximately the same widths while channel region 130has a greater width leading to channel region extensions 135 that extendbeyond the edges of the source and drain regions, i.e., beyond thechannel region's typical interfaces with each of the source and drainregions. Channel region extensions 135 are typically formed by anextension of the silicon active area from which the source, drain, andchannel regions are formed, or by formation of mesa regions immediatelyadjacent to the channel region. Because of their role in reducing theparasitic effects associated with TID, these extended active areastructures can be referred to as parasitic isolation devices (PIDs).

Each of channel region extensions 135 have a width x and a length y(FIG. 1E). Although FIGS. 1B and 1D-H illustrate channel regionextensions 135 having approximately the same dimensions as each other,this need not be the case. Channel region extension length y istypically smaller than the length of polysilicon gate 150, while channelregion extension width x can be selected to provide the desired devicefeatures. The increased length of the channel edge caused by theextensions, i.e., and increase from y to 2x+y, serves to increase theeffective channel length of channel region 130 to a value greater thanthe case where the channel region terminates “flush” with source anddrain regions 110 and 120. By increasing the net channel edge length,channel region extensions 135 lead to significantly reduced OFF stateleakage due to the attenuation of parasitic transistor short channeleffects.

In a typical implementation, the PID structure effectively increases thelocal channel length by more than a factor of two. The geometry of thePID provides for selective doping within specific regions 140 as shownin FIGS. 1F-1H. FIGS. 1F and 1G show top views of device 100 includingchannel extension impurity regions 140, typically formed along thelength of channel region extensions 135. Channel extension impurityregions 140 are typically formed to have relatively high impurityconcentrations, and are introduced to overlap end portions of thechannel region extensions so that the impurity concentration of theseend portions is increased relative to the impurity concentration ofthose portions of the channel region 130 located between source region110 and drain region 120. For example, where the channel region has a p−impurity concentration, channel extension impurity regions 140 form apair of p+ channel stops. This relatively high impurity concentration ofthe channel stop insures that the parasitic sidewall threshold is higherthan any negative threshold shift which might be induced by ionizingradiation. These more heavily doped extension impurity regions aretypically spaced apart from the endwall edges of source and drainregions 110 and 120 by respective portions of the extension regions ofthe same doping concentration as channel region 130 itself, so that themore heavily doped channel extension impurity regions do not form p+/n+junctions with the source and drain regions 110 and 120. Such junctionscan have very low breakdown voltages, thereby adversely effecting deviceperformance. The extension width x is generally determined by the choiceof doping ion and the manufacturing process's thermal diffusionexposure. For example, a boron doped PID structure would typically havea larger extension width x as compared to an Indium doped PID structure.Thus, extension width will typically be driven by concerns about excessback-diffusion and enhanced doping of the transistor active area. Ingeneral, either acceptor or donor impurities can be used to form channelextension impurity regions 140. Variation of impurity type, implantenergy, and dose will typically provide different types and/or levels ofradiation isolation improvement.

As shown in FIG. 1G, channel region extensions 135 are located below(and hidden by) gate electrode 150, which allows self-alignedconventional processing to be used subsequent to PID formation. In thecase of n-channel devices, the channel region extensions when combinedwith impurity doping can be used to prevent unwanted electricalinversion (threshold voltage depletion) of the edges (or sides) of then-channel transistor silicon channel area. This sensitive regiongenerally extends below the gate electrode between source region 110 anddrain region 120, as illustrated in FIG. 1H which shows an end view ofthe device. Thus, in some embodiments, channel extension impurityregions 140 are formed to extend to a depth greater than the depths ofthe source and drain regions. The PID can likewise be used in p-channeltransistors to prevent threshold voltage enhancement (opposite ofn-channel) caused by positive charge accumulation along the active areaedge of the PMOS transistor active area. Note that because the PIDstructure can increase the size of the transistors active area, gatecapacitance may also increase. This can lead to reduced speed inoperation of the transistor. To compensate for this effect, variousembodiments can adjust different aspects of the device geometry. Forexample, in one embodiment the width of the PMOS can be increased tohelp reduce the gate capacitance.

Thus, the PID structure reduces or eliminates parasitic field transistorinversion which reduces or eliminates a major source of I_(off) leakagecaused by charge build up at the silicon/SiO₂ (160) boundary. Moreover,the PID structure doped region provides for local minority carrierlifetime reduction, reduced sensitivity to substrate noise and bodyvoltage effects, and enhances the transistor snap-back voltage. The PIDstructures described herein can be fabricated in both bulk silicon andsilicon films, such as silicon-on-insulator (SOI).

Latch-up generally, and in the case of circuits in radiationenvironments SEL, is a serious failure mode in CMOS circuits. In thebest case, latch-up results in soft failure with a loss of data or logicstate. In the worst case, latch-up causes a destructive hard failurewith permanent loss of the circuit. Thus, from a circuit reliabilityperspective, latch-up is to be avoided at all costs. As isolation widthsshrink, device structures become even more susceptible to both latch-upfailure modes. Prevention of both transient and destructive failures isvery important in advanced CMOS IC's since most conventional CMOStechnologies have largely converged on the use of p− bulk substrates. Asnoted above, radiation environments present special problems to CMOScircuits in that high-energy particles deposit significant electricalcharge to the bulk substrate. The instantaneous appearance of staticcharge deposited by a high-energy particle passing through an IC canimmediately induce large displacement currents on the pico-second timescale accompanied with rapid potential shifts away from initial logicstates. The deposition of energy by the particle typically causes diodesto forward bias, followed by large transient injection currents whichreinforce the transient upset and can cause the CMOS circuit tolatch-up.

Latch-up in a conventional CMOS inverter circuit is illustrated in FIG.2. FIG. 2 shows a cross-section diagram of a CMOS inverter formed on ap− substrate. An equivalent circuit diagram is also shown in FIG. 2overlaid on the CMOS inverter. In CMOS technology, the basic switchingcircuit is an inverter formed from a complementary pair of MOStransistors, one NMOS and one PMOS. Electrical isolation is achieved byusing both dielectric and pn junction diodes. P-type doped regions(p-wells) isolate NMOS transistors, while n-typed doped regions(n-wells) isolate PMOS transistors. FIG. 2 illustrates the parasiticelectrical elements, bipolar transistors, resistors and diodes, thatco-exist within the CMOS structure. The source and drain regions of eachMOSFET in the inverter form bipolar emitters. In normal operation, thecircuit maintains a reverse bias of the diodes formed by then-well/p-substrate, the p+/n− junction of the PMOS and the n+/p−junction of the NMOS. External factors such as leakage, circuitswitching noise, particle upset, and transient overvoltage at the p+diffusion or undervoltage at the n+ diffusion can trigger one of both ofthe parasitic bipolar devices into the active state. If either of theemitter or base junctions is forward biased, emitter currents will besourced to corresponding base/collector regions, immediately shiftingthe local potential from its initial voltage state.

The network parasitic devices forms a pnpn silicon controlled rectifier(SCR) power device which can be unintentionally biased into a highcurrent, low impedance state, thereby causing latch-up. The circuitelements R_(S1), R_(S2), and R_(S3) are the net effective bulk p−substrate resistors, while R_(NW1) is the n-well series resistance andR_(PW1) is the p-well series resistance. In conventional technologysubstrate resistors would have resistances on the order of 1 kΩ orgreater. In a typical example of CMOS circuit operation, the bulksubstrate represents ground potential (0 volts) and is referred to asV_(SS). If current is injected from any source available to the p−substrate, the local potential (within the p− substrate) will increasein magnitude above zero volts as a function of the high value substrateresistance. The n+/p− diffusion diodes located in the p-well, and whichwere previously undisturbed, would then enter a forward biasedconduction as the P− substrate potential increases above approximately0.3 volts. It is this second contribution by the previously undisturbedn+/p− diffusion diodes which now positively enhances the initial smallsignal upset event, and can lead to latch-up. Likewise the same event ispossible starting with forward biased p+/n− diffusion diodes which arelocated within the n-well isolation doping region that is caused byeither an over voltage of the p+ diffusion or a transient under voltagecondition existing within the n-well local potential, which typicallyequals the circuit supply voltage or terminal voltage (V_(DD), orV_(CC)).

Thus, the network of series resistors plays a direct role in thetransient bias states of the different impurity doped regions, whichthen directly influences the bias and conduction state of the diodes.Latch-up can be prevented by keeping the diodes in their reversed bias(off) states. A first step in reducing or preventing latch-up isreducing the effective resistance of p− substrate resistors R_(S1) andR_(S2). The p− substrate resistance can be reduced using a high-doseburied layer (HDBL) 300 as illustrated in FIG. 3.

In the example of FIG. 3, HDBL layer 300 is a heavily doped region ofthe same conductivity type as the p− substrate, and is placed beneaththe active device regions of the device by ion implantation. In general,both the doping and damage characteristics of HDBL influence deviceperformance. HDBL structures are easily integrated into standard CMOSprocessing with the addition of typical steps such as, an implant step,a rapid thermal anneal (RTA), and an optional cleaning step in betweenthe implant and anneal. The process steps associated with the HDBL canbe performed either before or after dielectric isolation (SiO₂)formation either by LOCOS or the use of a shallow trench isolation (STI)structure common in many CMOS processes. While a variety of differentimpurities can be used, the most common implants are boron implanted at1.0-2.5 MeV (for p-type substrates), and phosphorus at 1.5-3.5 MeV (forn-type substrates). If a wafer-normal to ion implant beam angle of zerodegrees is set during ion implantation, the necessary energy to achievea desired depth of the ion implant can be significantly reduced.Implanted doses typically vary between 1×10¹³ and 5×10¹⁵ cm⁻², whichforms a highly doped and abrupt layer approximately 1.0-5.0 μm below thesilicon surface. The buried layer peak doping concentration can rangefrom 1×10¹⁷ cm⁻³ to 5×10²¹ cm⁻³. The buried layer is typically implantedsufficiently deep so that the final dopant profile (after all thermalprocesses) does not counterdope the retrograde well of oppositeconductivity, which results in low well-substrate breakdown voltage. Forexample, a CMOS twin well process includes an HDBL structure (i.e.,boron with an implant energy of 1.7 MeV and a dose of 1×10¹⁵ cm² forminga blanket p+ buried layer). The 1-D doping profiles after all thermalprocessing steps are completed are shown for the p+/n-well region andthe V_(SS) tie region in FIGS. 12 and 13, respectively.

For example, in the case of FIG. 3 where the substrate is p− doped, thedeep p+ layer provides a low resistance shunt 300 that significantlyreduces the series resistance between the pnp subcollector (below then-well) and the p+ collector (V_(SS)). Forming a p+ shunting layer 300via implantation also gives circuit designers the option of patterningthe implant to independently optimize substrate resistivities indifferent areas, e.g., for a system-on-chip design. Thus, substrateresistors R_(S1) and R_(S2) are now significantly reduced, typically bymore than two orders of magnitude, because of the presence of HDBL 300.The spreading resistance of the region can be lowered to less than 50Ωas compared to 2 kΩ or more for conventional CMOS isolation. However,use of the HDBL alone does not effect other parasitic resistances suchas R_(S3) and R_(PW1). Consequently, significant vertical resistancecontinues to exist.

A second step in addressing latch-up and other substrate currenttransients is to reduce the significant vertical resistance. Addition ofa vertical conductor 310 extending between the buried layer 300 and ap-well contact region (as shown) or some other substrate surfaceterminal significantly reduces this resistance. In one embodiment, thevertical conductor 310 is formed as a vertical impurity region havingthe same conductivity type as the substrate, but typically having animpurity concentration greater than that of the substrate. For example,where the substrate is a p− substrate, vertical conductor 310 can beformed from one or more p+ implants into the region extending betweenp-well contact 320 and buried layer 300. High-energy ion-implantationcombined with photoresist masking and/or multiple energy implantationcan provide a vertical p+ doped region with high aspect ratio dopingprofile to bridge the uplink path from an ohmic V_(SS) surface contactor Schottky diode to buried layer 300 or other p+ bulk layers as istypical for p− epi on p+ bulk substrates. For example, in oneimplementation vertical conductor 310 is formed by multiple ion-implantsteps at one or more energy levels.

When buried layer 300 and vertical conductor 310 are used in conjunctionwith each other, a new structure referred to as a buried guard ring(BGR) is formed. The BGR offers several isolation attributes. Since thelow resistance deep buried layer is now locally connected via a verticaldoping region or other conductor: (1) minimum impedance is provided formost or all of the accumulated, deposited or injected charge occurringwithin the silicon substrate region; (2) transient times for chargetermination are reduced thereby improving overall isolation by reducingor minimizing the duration of any n+/p− diode forward biasing whichreduces injected currents; (3) the BGR forms a low-resistance circuitleg back to the p-well contact terminal, thereby creating a currentdivider which effectively shunts and sinks a majority of any injected ordeposited current away from the R_(S4) and R_(S5) resistor legs, therebysignificantly reducing base current feedback to the parasitic npndevices shown and limiting voltage transients for the p-bulk siliconregion associated with the CMOS structure. The BGR structure, byeffectively shunting injected current or stray charge promptly to theV_(SS) terminal, reduces or prevents voltage transients which couldsubsequently forward bias either or both of the n−/p− diodes(n-well/p-substrate diode inherent in CMOS twin well isolation) and then+/p− diodes (inherent in NMOS transistor) and eliminates subsequenttriggering of the pnpn SCR network. Latch-up with the BGR isolationstructure present then becomes difficult or impossible to initiate,thereby preserving the circuit from ever entering destructive latch-up.

FIG. 4 illustrates and alternate implementation of the BGR structure. Inthis example, buried layer 400 is connected to p-well contact 420 usingan extended p-well area 410 as the vertical conductor. Such an approachcan potentially simplify device fabrication by making formation of thevertical conductor part of the normal p-well implant (or diffusion)process. Since the typical depth of buried layer 400 is from 1 to 5 μm,the p-well formation technique used should be capable of providing anadequately deep extended p-well. Moreover, extended p-well area 410 willtypically extend well into buried layer 400 to ensure proper electricalcontact.

In still other examples, the vertical conductor could be formed much theway vias or other vertical conductors are formed. For example, a deeptrench that terminates at or near the top of a buried layer could beused. With appropriate spacer formation, silicide formation, and fillingwith polysilicon or metal, such a conductor could extend to thesubstrate surface where it would be capped in a conventional manner andterminated as appropriate in a metallization scheme. Conductor formationcan also utilize emerging technologies, such as atomic layer deposition(ALD), or any other techniques as are well known by those having skillin the art.

FIG. 5 illustrates an example of direct connection using a metallizationscheme to contact HDBL horizontal layer 500. In this example, shallowtrench isolation (STI) structures are used to insulate the verticalconductor from the surrounding p-well and substrate. In a conventionalSTI structure, a shallow trench is etched into the substrate. The trenchis then thermally oxidized and filled with a deposited oxide to make itnon-conducting. Such a structure provides good isolation betweenadjacent MOS devices. However, in the example of FIG. 5, STI structure505 is extended down to buried layer 500. Instead of filling the STIstructure with an insulative material, STI structure 505 includes anohmic fill (e.g., polysilicon and/or metal) to provide the neededvertical conductor 510. Electrical contact between vertical conductor510 and buried layer 500 can be further enhanced using an ohmic contactimplant 507. Ohmic contact implant 507 is typically an ion implantextending from the bottom of the STI trench well into the buried layer.P-well contact 520 is formed so as to be electrically coupled tovertical conductor 510.

FIGS. 6A-6B illustrate other implementations of the BGR structure. InFIG. 6A, BGR structure 600 (formed using any of thestructures/techniques described above) and the CMOS inverter are formedin p− epitaxial layer 630 which in turn is formed on n+ or n− bulksubstrate 640. As will be well known to those having ordinary skill inthe art, various different techniques can be used to form epitaxiallayer 630. Similarly, FIG. 6B illustrates a BGR structure 650 (again,formed using any of the structures/techniques described above) formedalong with the CMOS inverter in p− epitaxial layer 680. Epitaxial layer680 is in turn formed on p+ bulk substrate 690.

FIGS. 7A-7C illustrate still other embodiments and variations of thebasic BGR structure. As shown in FIG. 7A, the BGR structure includesonly a vertical conductor 700 (e.g., a vertical doping structure,deposited conductor, etc.), and does not include the HDBL describedabove. Such an implementation can provide adequate device benefits insome cases, particularly where the CMOS devices are very closely formed,such as in an SRAM implementation. In this example, BGR structure 700extends to the p+ or p− bulk silicon substrate 720 while passing throughp− epi layer 710, as would be used, for example, in a p− epi on p+ bulkwafer. Epitaxial layer 710 is in turn formed on substrate 720 andcontacted using only the vertical BGR extension 700. FIG. 7B illustratesa similar implementation, where vertical BGR component 730 isimplemented into p− bulk silicon. Similarly, FIG. 7C shows animplementation where vertical BGR component 750 is formed in an epilayer 760 which in turn has beneath it a buried oxide layer 770 (orother SOI implementation) formed using well known SOI techniques.

FIGS. 8A-8E illustrate still other embodiments and variations of thebasic BGR structure. As shown in FIG. 8A, BGR structure 800 and acorresponding CMOS inverter have the same basic structure as thatillustrated in FIG. 3. Note that in this example, as well as theexamples of FIGS. 8B-8E, the BGR structure can in general be formedusing any of the structures/techniques described above. However, ann-type doping layer 805 is included beneath the buried layer portion ofBGR 800 to act as a carrier recombination and/or gettering layer. Layer805 is typically left floating, i.e., it is not electrically coupled toground or some other reference voltage. However, in otherimplementations, layer 805 can be coupled to a suitable potential.N-doped layer 805 can be formed using any number of implant or diffusiontechniques, as will be well known to those having ordinary skill in theart.

As shown in FIG. 8B, devices using the BGR structure can implemented insilicon on insulator (SOI) substrates in order to take advantage of theunique properties of such substrates. In this example, BGR structure 810and its associated CMOS devices are formed in epitaxial layer 815.Buried oxide layer 820 is formed in (e.g., using a separation byimplantation of oxygen (SIMOX) process) or on (e.g., using a bondedwafer process) the bulk silicon wafer. Thus, BGR devices can furthertake advantage of the beneficial properties of SOI wafers including:reduced parasitic capacitance, additional SEU immunity, and in somecases simplified processing.

FIG. 8C illustrates an embodiment similar to that of FIG. 8B. However,in this case BGR structure 825 is formed from a single verticalconductive component as described above. BGR structure 825 typicallyextends through the epi layer to approximately the depth of buried oxidelayer 830. In some embodiments, BGR structure 825 can stop well shortof, or extend into buried oxide layer 830.

FIG. 8D illustrates still another example where the BGR structure isused with a CMOS device that includes triple well isolation, common formany RF devices. Here, BGR structure 835 is formed within the p−substrate region 840 of the isolated p-well common to triple wellisolation implementations, but still surrounded (on the sides) by thedeep n-well structures and (below) by a buried n-layer used to providethe additional isolation.

Finally, FIG. 8E illustrates still another BGR embodiment where BGRstructure 845 is formed from a single vertical conducting structure andno HDBL. BGR structure 845 is implemented in the isolated p-well 850 ofthe triple well isolation structure. The final depth of the vertical BGRlayer is consistent with the depth of the isolated p-well region and istypically less than 2 microns.

FIGS. 9A-9B illustrate a 2D simulation (using the Taurus-Medici devicesimulation tool from Synopsys, Inc.) of the doping profile of a CMOStwin well structure with the heavily p+ doped region now extending belowthe V_(SS) terminal to the p+ buried layer. FIG. 9A shows the BGRstructure emphasizing its connection to the V_(SS) terminal, while FIG.9B illustrates portions of the BGR structure under the device n-well.Contour 900 shows the 1×10¹⁸ cm⁻³ doping contour generally indicatingthe region of p+ doping that forms the BGR. Reference numbers 300, 310,and 320 show the buried layer, vertical conductor, and p-well contact320 respectively. The impurity concentration of the BGR regionsgenerally varies between 1×10¹⁸ and 1×10¹⁹ cm⁻³. FIG. 9C shows aone-dimensional doping profile from V_(SS) (i.e., below the p+ ohmiccontact in the p-well) to the p+ buried layer. In general, the p+vertical doping region is located only below the V_(SS) terminal (thep-well connection region) so that it does not interfere with normalcircuit operations. As will be described below in connection with FIG.11, the improved contact to the HDBL shunt layer is shown to beextremely effective in preventing latch-up triggering.

FIG. 10 provides a typical current vs. voltage (I-V) trace illustratinglatch-up as well as terms and features normally used in conjunction witha latch-up I-V trace. In this example, the I-V characteristics of a p+diffusion experiencing overvoltage are shown. The voltage at the p+diffusion starts at V_(DD) (1.5 V) and is ramped to higher voltage aswould be the failure mode of an overvoltage condition. Since the p+diffusion is diode isolated from the n-well, it can only block currentup to the forward bias diode built-in potential of approximately +0.3 Vbefore becoming active. For overvoltages greater than 0.3 V higher thanV_(DD), (V_(BE)) current is injected across the p+/n− emitter/base diodeand collected in the base/collector junction. Once active, the pnpbipolar parasitic transistor actively injects current into the emitterbase junction. The p+ emitter current is divided and flows to both then-well base contact and the p+ collector (p-well). As the SCR triggercurrent is approached, pnp collector current is flowing to the V_(DD)terminal within the n-well, and significant hole current is flowing tothe V_(SS) terminal which represents the pnp collector terminal. Notethat prior to reaching the SCR trigger current, the n+/p− diode (whichrepresents the emitter/base junction of the npn parasitic transistor)remains in the off state since, based on its local potential, it isstill in a reversed biased blocking state.

If the overvoltage transient condition persists the vertical pnp (VPNP)will continue to inject holes into the p-substrate, which is terminatedat the V_(SS) (ground) body tie. The VPNP collector current quicklydrives the local potential in the p-well up from zero volts to +0.3 Vabove V_(SS), which then forward biases the n+/p− diode since the n+diffusion potential is fixed at Vss. This n+/p− diode, which has beeninactive so far, now begins to inject electrons into the p− substrate asthe potential of the p-well continues to increase. The electrons(minority carriers) injected into the p-well will be collected at theCMOS n-well, since this is the highest available local potential node.The n-well is now acting as an npn sub-collector. The lateral npn (LNPN)collector current now provides significant current to the n-well base ofthe VPNP. LNPN collector current flows across n-well series resistors(FIG. 3). NPN collector current now flowing in the n-well forces anadditional potential drop within the n-well (at the p+ diffusion) tosome voltage below V_(DD). The point on the I-V plot in FIG. 10 thatmarks the transition of the pnpn from the high impedance (blocking)state to the negative differential resistance state is called thetrigger current. Trigger current is a meaningful term by which to judgelatchup resistance capability since it is a measure of the currentnecessary to shift the “net potential” of the two independent diodes by+0.7 V. It is always desirable that the trigger current be as high aspossible to prevent the pnpn from forward biasing to the low impedancenon-blocking state.

A transitional phase or negative resistance occurs once the triggerpoint is passed and the pnpn network transitions from the low currentblocking state to the high current, low impedance state. Finally, theI-V curve reaches its minimum voltage value (vertical portion of FIG.10). Just after the trigger point is passed, the current flowing betweenthe two voltage rails (V_(DD) and V_(SS)) moves from the substrate andn-well resistor to the surface, where the resistance is lowest. Eventhough the sheet resistances of the n-well and p-well regions along theedges of the STI are high, the spatial separation between the p+diffusion (V_(DD)) and the n+ diffusions (V_(SS)) is small. The currentflowing between V_(DD) and V_(SS) has now reached the lowest networkresistance, and the associated voltage drop also reaches its minimumvalue. The aforementioned shunt resistors determine how much current isretained in the network resistor legs, and any current remaining here issubtracted from the base currents of the VPNP and LNPN, decoupling thebipolar effectiveness. Latch-up that reaches this saturation stage willcause overheating and can melt both the silicon and metal regions fromthe heat generated by the large currents being passed. The holdingvoltage is the minimum voltage for which positive current feedback canbe sustained by the VPNP and LNPN pair.

With this in mind, FIG. 11 illustrates the significant improvements madepossible when BGR techniques are applied to a conventional CMOSisolation scheme. Several curves are illustrated. The lower curve 1100is a typical latch up I-V trace for an advanced CMOS technology withadvanced n+/p+ spacing of 270 nm. The I-V trace shows that snap-back(latch-up failure) has occurred at approximately 200 μA/μm, and thelatch up holding voltage is approximately 1.2 V. The second curve 1110shows marked improvement associated with the presence of an HDBL, withsnap-back now >500 μA/μm and holding voltage approximately 1.4 V.However, in both cases latch up has occurred with a holding voltage thatis below the power supply of 1.5 V. The remaining curves 1120 whichrepresent IV characterization in the presence of a BGR structure showthat no triggering event has occurred for even smaller n+/p+ spacing of220 nm and with over voltage currents >1.6 mA/μA/μm. The lack oflatch-up snap-back at these currents marks an improvement of more than 3orders of magnitude. The simulation data shows the effectiveness of theBGR structure in preventing latch-up, even at extremely aggressive 220nm n+/p+ spacing.

The three dimensional heavily doped BGR layers extend both horizontallyand vertically thus creating low ohmic regions of contiguous and/orconnected to isolation doping regions which stand separated from thetransistor doping regions. As can be seen in FIGS. 3 and 9A-9B, the BGRstructure can be integrated into any existing CMOS twin well isolationstructure with no adverse impact to existing structures, thus enablingthe improvements necessary to provide robust isolation for bothconventional circuit noise initiated latch up and single event latch up.Moreover, BGR structures and techniques can be used to prevent latch-upin both radiation-hardened and conventional microelectronic circuits.BGR structures and techniques can also be implemented on a variety ofdifferent process variations, such as epitaxial silicon on bulk silicon(either with or without a heavily doped substrate), SOI, SOS, and thelike.

The BGR structures and techniques can also be used to reduce oreliminate both single-event upset (SEU) and single-event transient (SET)events. SEU and SET can be caused by a high-energy particle, e.g., asingle heavy ion or nuclear particle such as a neutron or alphaparticle, passing through a critical node in an IC. Immediately afterbeing struck by such a particle electrons and holes will be separatedfrom the silicon lattice as a function of the particle's energy which isexpressed as the linear energy transfer (LET Mev/mg-cm) of theparticular particle. If the charge deposited and ultimately collected asfree electrons and holes is greater than the critical charge of a memorycell or some other state-related device, a single-event upset can occur.The susceptibility of ICs to single-event upsets typically depends onthe amount of critical charge required to “flip” a bit and theprobability that a particle with a LET large enough to deposit thatcritical charge will strike a sensitive node. The production of largenumbers of electron/hole charges also creates a potential dipole withinthe particle track passing through the silicon material. The Hall effectsegregates the holes and electron charges to opposite sides of theelectrostatic field. Some electrons and holes can recombine in thelattice via SRH and Auger mechanisms. However, as a result of theirhigher mobility, electrons are quickly collected at the positiveterminals, whereas the net concentration of holes, which have lowermobility and remain with the silicon body as static charge whileunrecombined because of depleted electrons. The positive charges causethe local potential of the p-substrate to increase in voltage to apositive value and continues to influence the local potential of the p−substrate positively for several nanoseconds after the high energyparticle strike. As a result of the potential upset within the p−substrate, other unstruck diodes are now activated which createssecondary currents and enhances the initial upset more significantly, aneffect often referred to as single event transient (SET). While softerror results in data corruption, it is not destructive to the physicalcircuit.

Since CMOS logic typically uses inverter gates and cross coupled logicgates to store binary bits in any digital circuit, the aforementionedBGR devices and techniques will prove useful in reducing SEU and SET.The BGR structure serves a similar role with regards to soft error as inthe latch-up isolation. In both cases the BGR structure effectivelysinks excess positive charge generated by the upset event, whilemaintaining node potential, and limiting the transient voltage swing andduration following an SEU, SEL, or SET event. Process and devicesimulations are used to quantify the effectiveness of the BGR structureusing what is known as mixed-mode simulation. Test cases for SEU upsethave been simulated for heavy ions (krypton) with LET of 110 Mev/mg-cm.In these simulations, where an inverter is brought to either a digital“one” state or a digital “zero” state and the mixed-mode SEU simulationis executed. In 21 different cases (21 different ion strike points, orangles of strike), the conventional CMOS inverter was shown to fail,i.e., an SEU occurred 9 times out of 21, or 43% of the time. In thesimulations, the SEU charge transfer is completed within by 50 ps of theevent, however the simulation was continued to 1 ns to observe chargerecovery and circuit response.

Results from the same simulations using a CMOS inverter with a similarstructure but now including the aforementioned BGR structure show markedimprovement. In this example, there are no failures in the same 21cases. While the BGR structure demonstrated no failure in 21 SEU tests,the contrast with the bulk silicon simulations is even more significant.In several cases of the BGR structure simulations, significantimprovement both in voltage stability and transient response were seen.The BGR structures, when compared to the bulk example, appear to “clamp”the node potential for certain SEU strikes and completely eliminates thepossibility of secondary injection by the unstruck nodes since the p−substrate potential is maintained at or very near zero during SEUevents. Moreover, simulation studies indicate that the BGR structurewould significantly improve SEU and SET soft error when compared toconventional CMOS without BGR protection. The simulation studies alsoindicate that BGR effectiveness in reducing SEU events depends at leastin part on the net resistance of the structure. For example, reducingthe resistance of the HDBL in the BGR improved performance of thestructure. Such resistance reduction can be optimized, for example, byusing shallower HDBL implants, by increasing the implant dose, and/or byoptimizing the vertical doping profile.

Regarding terminology used herein, it will be appreciated by one skilledin the art that any of several expressions may be equally well used whendescribing the operation of a circuit including the various signals andnodes within the circuit. Any kind of signal, whether a logic signal ora more general analog signal, takes the physical form of a voltage level(or for some circuit technologies, a current level) of a node within thecircuit. Such shorthand phrases for describing circuit operation usedherein are more efficient to communicate details of circuit operation,particularly because the schematic diagrams in the figures clearlyassociate various signal names with the corresponding circuit blocks andnodes.

An insulated gate field effect transistor (IGFET) may be conceptualizedas having a control terminal which controls the flow of current betweena first current handling terminal and a second current handlingterminal. Although IGFET transistors are frequently discussed as havinga drain, a gate, and a source, in most such devices the drain isinterchangeable with the source. This is because the layout andsemiconductor processing of the transistor is frequently symmetrical(which is typically not the case for bipolar transistors). For anN-channel IGFET transistor, the current handling terminal normallyresiding at the higher voltage is customarily called the drain. Thecurrent handling terminal normally residing at the lower voltage iscustomarily called the source. A sufficient voltage on the gate(relative to the source voltage) causes a current to therefore flow fromthe drain to the source. The source voltage referred to in N-channelIGFET device equations merely refers to whichever drain or sourceterminal has the lower voltage at any given point in time. For example,the “source” of the N-channel device of a bi-directional CMOS transfergate depends on which side of the transfer gate is at the lower voltage.To reflect this symmetry of most N-channel IGFET transistors, thecontrol terminal may be deemed the gate, the first current handlingterminal may be termed the “drain/source”, and the second currenthandling terminal may be termed the “source/drain”. Such a descriptionis equally valid for a P-channel IGFET transistor, since the polaritybetween drain and source voltages, and the direction of current flowbetween drain and source, is not implied by such terminology.Alternatively, one current-handling terminal may arbitrarily deemed the“drain” and the other deemed the “source”, with an implicitunderstanding that the two are not distinct, but interchangeable. Itshould be noted that IGFET transistors are commonly referred to asMOSFET transistors (which literally is an acronym for“Metal-Oxide-Semiconductor Field Effect Transistor”), even though thegate material may be polysilicon or some material other than metal, andthe dielectric may be oxynitride, nitride, or some material other thanoxide. The casual use of such historical legacy terms as MOS and MOSFETshould not only be interpreted to literally specify a metal gate FEThaving an oxide dielectric.

Regarding power supplies, a single positive power supply voltage (e.g.,a 1.5 volt power supply) used to power a circuit is frequently named the“V_(DD)” power supply. In an integrated circuit, transistors and othercircuit elements are actually connected to a V_(DD) terminal or a V_(DD)node, which is then operably connected to the V_(DD) power supply. Thecolloquial use of phrases such as “tied to V_(DD)” or “connected toV_(DD)” is understood to mean “connected to the V_(DD) node”, which istypically then operably connected to actually receive the V_(DD) powersupply voltage during use of the integrated circuit. The referencevoltage for such a single power supply circuit is frequently called“V_(SS).” Transistors and other circuit elements are actually connectedto a V_(SS) terminal or a V_(SS) node, which is then operably connectedto the V_(SS) power supply during use of the integrated circuit.Frequently the V_(SS) terminal is connected to a ground referencepotential, or just “ground.” Generalizing somewhat, the first powersupply terminal is frequently named “V_(DD)”, and the second powersupply terminal is frequently named “V_(SS).” Historically thenomenclature “V_(DD)” implied a DC voltage connected to the drainterminal of an MOS transistor and V_(SS) implied a DC voltage connectedto the source terminal of an MOS transistor. For example, old PMOScircuits used a negative V_(DD) power supply, while old NMOS circuitsused a positive V_(DD) power supply. Common usage, however, frequentlyignores this legacy and uses V_(DD) for the more positive supply voltageand VSS for the more negative (or ground) supply voltage unless, ofcourse, defined otherwise. Describing a circuit as functioning with a“V_(DD) supply” and “ground” does not necessarily mean the circuitcannot function using other power supply potentials. Other common powersupply terminal names are “V_(CC)” (a historical term from bipolarcircuits and frequently synonymous with a +5 volt power supply voltage,even when used with MOS transistors which lack collector terminals) and“GND” or just “ground.”

Moreover, implementation of the disclosed devices and techniques is notlimited by CMOS technology, and thus implementations can utilize NMOS,PMOS, and various bipolar or other semiconductor fabricationtechnologies. While the disclosed devices and techniques have beendescribed in light of the embodiments discussed above, one skilled inthe art will also recognize that certain substitutions may be easilymade in the circuits without departing from the teachings of thisdisclosure. Also, many circuits using NMOS transistors may beimplemented using PMOS transistors instead, as is well known in the art,provided the logic polarity and power supply potentials are reversed. Inthis vein, the transistor conductivity type (i.e., N-channel orP-channel) within a CMOS circuit may be frequently reversed while stillpreserving similar or analogous operation. Moreover, other combinationsof output stages are possible to achieve similar functionality.

Although the present invention has been described with respect to aspecific preferred embodiment thereof, various changes and modificationsmay be suggested to one skilled in the art and it is intended that thepresent invention encompass such changes and modifications that fallwithin the scope of the appended claims.

What is claimed is:
 1. A semiconductor device comprising: a substratehaving, at a surface thereof, a region of a first conductivity type anda first substrate impurity concentration; a first well of the firstconductivity type formed at the surface of the substrate, and having afirst well contact region within the first well; a second well of asecond conductivity type opposite the first conductivity type formed atthe surface of the substrate; a first transistor having a drain regionand a source region disposed within the first well; a buried layer ofthe first conductivity type extending beneath the first well and havinga buried layer impurity concentration greater than the first substrateimpurity concentration, said buried layer disposed beneath and separatedfrom the source and drain regions, and having a depth that is shallowerthan that of the second well; and a conductive region disposed betweenthe buried layer and the first well contact region, said conductiveregion laterally spaced and separated from the source and drain regions,and said conductive region providing a higher conductance between theburied layer and the first well contact region than otherwise providedin the absence of said conductive region.
 2. The semiconductor device ofclaim 1 further comprising: a buried insulating layer extending beneaththe first and second wells, wherein the buried layer depth extendssubstantially to the buried insulating layer.
 3. The semiconductordevice of claim 2 wherein the buried insulating layer comprises aninsulating substrate.
 4. The semiconductor device of claim 1 wherein:the buried layer extends beneath the first well, but not beneath thesecond well.
 5. The semiconductor device of claim 4 further comprising asecond buried layer of the second conductivity type disposed below theburied layer of the first conductivity type within the first well, saidsecond buried layer disposed at a depth to intersect the second well. 6.The semiconductor device of claim 1 wherein the conductive regionfurther comprises a conductance over its entire vertical extentexceeding that corresponding to a p-type impurity region having animpurity concentration greater than 1×10¹⁸ cm⁻³.
 7. The semiconductordevice of claim 1 wherein the conductive region extends vertically fromthe buried layer to the first well contact region.
 8. The semiconductordevice of claim 1 wherein the substrate further comprises: a substratematerial; and an epitaxial layer of semiconductor material formed on thesubstrate material, wherein the epitaxial layer of semiconductormaterial includes the substrate region having the first substrateimpurity concentration, and wherein the first well, the buried layer,and conductive region are at least partially located in the epitaxiallayer of semiconductor material.
 9. The semiconductor device of claim 8wherein the substrate material is a semiconductor material having thefirst conductivity type and having an impurity concentration differentfrom the first substrate impurity concentration.
 10. The semiconductordevice of claim 8 wherein the substrate material is an insulatingmaterial.
 11. A method for making a radiation-hardened integratedcircuit (RHIC) product, said method comprising: providing a substratehaving, at a surface thereof, a region of a first conductivity type anda first substrate impurity concentration; forming, at the surface of thesubstrate, a first well of the first conductivity type, and having afirst well contact region within the first well; forming, at the surfaceof the substrate, a second well of a second conductivity type oppositethe first conductivity type; forming a drain region and a source regionfor a first transistor within the first well; forming a buried layer ofthe first conductivity type extending beneath the first well and havinga buried layer impurity concentration greater than the first substrateimpurity concentration, said buried layer disposed beneath and separatedfrom the source and drain regions, and extending to a depth that isshallower than that of the second well; and forming a conductive regiondisposed between the buried layer and the first well contact region,said conductive region laterally spaced and separated from the sourceand drain regions, and said conductive region providing a higherconductance between the buried layer and the first well contact regionthan otherwise provided in the absence of said conductive region. 12.The method of claim 11 further comprising: forming a buried insulatinglayer extending beneath the first and second wells, wherein the buriedlayer depth extends substantially to the buried insulating layer. 13.The method of claim 12 wherein the buried insulating layer comprises aninsulating substrate.
 14. The method of claim 11 wherein: the buriedlayer extends beneath the first well, but not beneath the second well.15. The method of claim 14 further comprising forming a second buriedlayer of the second conductivity type disposed below the buried layer ofthe first conductivity type within the first well, said second buriedlayer disposed at a depth to intersect the second well.
 16. The methodof claim 11 wherein the conductive region extends vertically from theburied layer to the first well contact region.
 17. The method of claim11 wherein said providing comprises: forming an insulating film on asubstrate material; and forming a semiconductor film on the insulatorfilm, wherein the semiconductor film includes the substrate regionhaving the first substrate impurity concentration, and wherein the firstwell, the buried layer, and the conductive region are disposed withinthe semiconductor film.
 18. The method of claim 11 wherein saidproviding comprises: forming an epitaxial layer of semiconductormaterial on a substrate material; wherein the epitaxial layer ofsemiconductor material includes the substrate region having the firstsubstrate impurity concentration, and wherein the first well, the buriedlayer, and conductive region are at least partially located in theepitaxial layer of semiconductor material.
 19. The method of claim 18wherein the substrate material is an insulating material.
 20. The methodof claim 19 wherein the buried layer depth extends substantially to theinsulating substrate material.